#define _CRT_SECURE_NO_WARNINGS 1

//https://www.luogu.com.cn/problem/P1063
using namespace std;
#include <stdio.h>
#include <math.h>
int main(void)
{
    int a, b, c;
    double s, S;
    scanf("%d %d %d", &a, &b, &c);
    s = (a + b + c) / 2.0;
    S = sqrt(s * (s - a) * (s - b) * (s - c));
    printf("%.3lf\n", S);
    return 0;
}

